That’s how it is now. Rather than increase the CAFE requirements(which would make people angry and requires government action) they did the back door method by having the EPA change the parameters of their fuel mileage tests. Cars have more horsepower nowdays and if you use that HP stupidly, your mileage goes down real fast. In other words, your mileage varies more depending on how you drive. That makes it easier for the EPA to get the mileage results they want to get by adjusting the parameters of the test.

Higher power would mean less throttle required to match the acceleration graphs on the test equipment ,, should mean higher mileage ... at least as far as a hemi 300c is concerned as the car is substantially unchanged... I don’t think the test has changed much ,, you should be able to make valid comparisons between current and prior years ... http://www.nhtsa.gov/fuel-economy